The series has been considered a high-water mark in fantasy publishing. Envisioned by the husband-and-wife team of Ian and Betty Ballantine, and edited by Lin Carter, it featured cover art by illustrators such as Gervasio Gallardo, Robert LoGrippo, David McCall Johnston, and Bob Pepper. The agreement signed between the Ballantines and Carter on November 22, 1968 launched the project. In addition to the reprints comprising the bulk of the series, some new fantasy works were published as well as a number of original collections and anthologies put together by Carter, and Imaginary Worlds, his general history of the modern fantasy genre.[1]

The series was never considered a money-maker for Ballantine, although the re-issue of several of its titles both before and after the series' demise shows that a number of individual works were considered successful. The Ballantines supported the series as long as they remained the publishers of Ballantine Books, but with their sale of the company to Random House in 1973 support from the top was no longer forthcoming, and in 1974, with the end of the Ballantines' involvement in the company they had founded, the series was terminated.[citation needed]

After the termination of the Adult Fantasy series, Ballantine continued to publish fantasy but concentrated primarily on new titles, with the older works it continued to issue being those with proven track records. In 1977, both its fantasy and science fiction lines were relaunched under the Del Rey Books imprint, under the editorship of Lester and Judy-Lynn del Rey. Carter continued his promotion of the fantasy genre in a new line of annual anthologies from DAW Books, The Year's Best Fantasy Stories, also beginning in 1975.[1] Meanwhile, the series' lapsed mission of restoring classic works of fantasy to print had been taken up on a more limited basis by the Newcastle Forgotten Fantasy Library, launched in 1973.[citation needed]

Ballantine published these fantasies and fantasy criticism before hiring Carter as consultant.[2] Some were labeled "A Ballantine Adult Fantasy" on the first Ballantine cover. Later reprints of some bore the Unicorn's Head colophon.

Two volumes published after retirement of the Unicorn's Head colophon were evidently intended for the series. The first has a Carter introduction and the second completes a set of four begun under his editorship.

Carter intended to reissue or compile these books for the series, according to statements in his introductions to other books in the series and lists discovered among his effects after his death or elsewhere.[4]